An increasing amount of oil and gas is originating from what are often referred to as “unconventional” subterranean oil reservoirs. “Shale reservoirs” are examples of such unconventional subterranean oil reservoirs. In unconventional oil reservoirs such as shale reservoirs, the oil recovery efficiency may be comparatively low due to the low permeability of the formation. This has resulted in the development of enhanced oil recovery schemes to stimulate production and increase recovery efficiency. It is desirable for such schemes to maximise the oil recovery efficiency.
An enhanced oil recovery scheme may involve the injection of an injection gas, e.g. hydrocarbon gas, carbon dioxide or steam, into the reservoir. The injection gas may be injected into fractures which have been induced in the formation surrounding the well, for example in the case of a fracture stimulated horizontal well. The presence of the injection gas maintains pressure in the reservoir and increases the mobility of the oil within the reservoir such that it more readily enters the well and the production conduit.
In some embodiments injection gas is injected into the reservoir via a first well and oil is produced in a second, adjacent, well. In other embodiments, a single well may be used to inject gas into the reservoir in a first phase and then produce oil in a second phase; this method is often referred to as a “huff and puff” process.
In order to maximise oil recovery efficiency, it is often desirable to inject the injection gas into the reservoir at a high pressure and then provide a period during which the injection gas diffuses within the reservoir where it can combine with the interstitial oil. This period if often referred to as a “soak” period and may last days, weeks or months. After the “soak” period, the well is reopened and the oil and gas enters the production flow path from the reservoir.
The production of oil from an unconventional oil reservoir during enhanced oil recovery schemes is generally governed and facilitated by the expansion of the oil within the formation as pressure is reduced in the wellbore and fractures, causing the oil to “seep” into the wellbore. The injection gas is designed to interact with the oil within the low-permeability formation and diffusion allows the oil to migrate into the well, and thus be produced, once the production phase begins.